In the high power laser system, optical elements having formed thereon an AR coating having a high laser damage threshold are used to efficiently take out a laser output. Such an AR coating is usually formed by treating the surface of glasses with an aqueous solution of acidic salt showing weak basicity in water. Corrosion of the glass surface with an aqueous solution of acidic salt showing weak basicity such as Na.sub.2 HAsO.sub.4 can be explained by the synergistic effect of two processes, one being a leaching process in which a somewhat porous layer, i.e., a so-called "Aoyake", is formed by the ion exchange reaction between alkali ions in the glass with H.sup.+ ions in the aqueous solution; and another being an etching process in which the Si-O-Si network of the glass is broken and the whole glass is dissolved.
If the leaching process is predominant, although a porous layer is formed, its porosity cannot increase sufficiently and, therefore, a satisfactory low refractive index layer cannot be formed. On the other hand, if the etching process is predominant, a porous layer is not almost formed and only the surface of glass is dissolved and, therefore, it is not possible to form a low refractive index layer.
It is known that if aluminum ion, Al.sup.3+, is added to the aqueous solution of acidic salt showing weak basicity, the leaching process and the etching process can be appropriately controlled.
When, however, optical glasses used in the laser system are treated with such Al.sup.3+ -containing weakly basic aqueous solutions, although a low refractive index coating is formed, the coating thickness becomes constant at a certain value and cannot be increased beyond the value even if the treating duration time, treating temperature, and Al.sup.3+ concentration are changed. That is, the coating thickness cannot be increased to such a level that the reflectivity becomes the minimum at 1.06.mu. which is a lasing wavelength of Nd.sup.3+ laser.
The present inventors have already disclosed in U.S. patent application Ser. No. 601,581, filed Apr. 18, 1984 that when a glass comprising, in % by weight, 65 to 75% SiO.sub.2, 5 to 20% alkali metal oxide, 0 to 15% B.sub.2 O.sub.3, 0 to 15% alkaline earth metal oxide, and 0 to 5% others is dipped in a treating solution prepared by further adding Si.sup.4+ ions to a mixed solution of a weakly basic solution and the Al.sup.3+ -containing solution, the Si.sup.4+ ions control the rate of etching of the glass surface, thereby lowering the reflectivity at any wavelengths and as a result, an AR coating which is a porous layer having a high laser damage threshold can be formed. However, this porous AR coating according to this method has a disadvantage in that the porosity on the surface is large and the coating has only such a mechanical strength that it is damaged by merely touching with a finger. This is inconvenient in assembling or handling the laser system.